1. Field of the Invention
The present invention relates to an apparatus and method for fabricating semiconductors, and more particularly, to an apparatus and method for measuring an aerial image from which the effects or influences of various defects on a photomask can be inspected.
2. Description of the Related Art
In general, an aerial image that is obtained from a photomask or a reticle (hereinafter commonly referred to as "photomask") is measured in order to inspect all types of defects exhibited on the photomask to determine the resulting influence of these defects on a pattern formed on the wafers using the photomask.
FIG. 1 is a schematic view of the configuration of a conventional aerial image measuring apparatus. The conventional apparatus for measuring an aerial image includes an optical source 2 for emitting light, e.g., deep ultraviolet (DUV) or I-line, a motor filter 4 having a filter suitable for a wavelength of light irradiated from the optical source 2, and a condenser lens 10 for condensing light that passed through an illumination aperture 6 and a vision aperture 8. The illumination aperture 6 controls the numerical aperture (NA) and the coherency of the light. The condensed light is then emitted to a surface of a photomask 50 opposite to the surface on which a chrome pattern 52 is formed.
A charge-coupled device (CCD) camera 30 forms an aerial image by converting light passing through the photomask 50 into an electrical signal. An aerial image measurement system (AIMS) 40 measures the aerial image. The light passing through the photomask 50 is transmitted to the CCD camera 30 via an objective lens 12, a tube lens 14, a 7.times.expanded projection lens 16 and an imaging aperture 20. An auxiliary lens 22 for observing the numerical aperture (NA) and the coherency is installed between the imaging aperture 20 and the CCD camera 30. Also, a CCD camera 15 for visible ray observation is installed to observe light focused on the tube lens 14 through the objective lens 12 using an auxiliary outputter (not shown) such as a monitor.
In the conventional aerial image measuring apparatus having such a configuration, light from the optical source 2 is irradiated to the surface of the photomask 50 opposite to the surface on which the chrome pattern 52 is formed so that an aerial image is measured using only the light passing through the photomask.
As a result, the conventional aerial image measuring apparatus can not inspect all types of defects which may exist on the photomask pattern so that the effects or influences of such defects on the photomask can not be accurately determined.
More specifically, the conventional aerial image measuring apparatus measures only an aerial image formed by the light passing through the photomask from the surface of the photomask opposite to the surface on which the chrome pattern is formed. Therefore, one cannot inspect the effects of numerous factors existing over the entire upper surface of the photomask on which the chrome pattern is formed. Such effects that should be inspected include: the reflectivity variation on the surface of an anti-reflection layer coated on the chrome pattern; chrome particles remaining at a repaired portion or its adjacent portion after a defect on the photomask is repaired; an ion beam source, e.g., gallium, in case that the chrome is removed by an ion beam; a damaged portion on the photomask generated after the defect on the photomask is repaired; the thickness variation of the chrome pattern layers formed on the photomask; and contaminants, such as flowable or unflowable particles existing on the chrome pattern formed on the photomask, or other organic materials generated during various processes.